As is known in the art of rechargeable batteries, referred to also as secondary storage cells, not all of the current supplied to the battery is recoverable, or utilized in the chemical reactions by which the battery is charged. The percentage of incremental input current, or charge, that is recoverable at any given point in the charge cycle is referred to as the charge acceptance of the battery. The cumulative charge accepted by the battery determines the battery state of charge (i.e., a percentage of full charge).
One technique for determining the state of charge of a rechargeable battery, described in U.S. Pat. No. 5,012,176, utilizes a mathematical model providing a relationship between charge acceptance and state of charge. Detection of the heat emanating from the battery during charging and the current entering the battery are used to calculate the state of charge which in turn, is used to terminate the battery charging process in order to avoid energy waste once a maximum state of charge is calculated.
As is also known, the power accepted by a rechargeable battery is equal to the input power multiplied by the charge acceptance. That portion of the input power that is not accepted is dissipated as heat. Due to the relatively high thermal resistance of batteries, generally, the input charging current supplied to a battery must be limited in order to prevent an over-temperature condition.
Another reason for limiting the input charging current is the inverse relationship between battery temperature and charge acceptance which generally dominates the battery behavior (i.e., unless the battery is charged so slowly that the thermal rise is minimal and can be ignored). That is, as the battery temperature rises, the charge acceptance is degraded, often resulting in an incapability of the battery to reach its fully charged state. For this reason, the battery temperature is sometimes measured and used to trigger a reduction in the charging current.
Regardless of the reason for limiting the input charging current to a battery, the result is that the battery takes an undesirably long time to charge. For example, it may be necessary to reduce the charging current from a level consistent with a 4C charging rate (i.e., by which the battery would be fully charged in fifteen minutes assuming 100% charge acceptance) to a C/10 rate (i.e., by which the battery would be fully charged in ten hours assuming 100% charge acceptance) in order to ensure that the steady state battery temperature rise does not exceed 8.degree.-10.degree. C.
Battery charging is often characterized as occurring in three phases, including an initial charging phase during which the charge acceptance is relatively high, an intermediate charging phase when the charge acceptance decreases, and a final charging phase during which the charge acceptance approaches zero percent and the battery has reached its full charge state. One type of conventional battery charger circuit decreases the level of input current supplied to a battery at the transition between such phases based on battery voltage. However, even with careful selection of the input current, this technique can only provide optimum charging (i.e., the fastest charging rate consistent with safe battery temperature) during a portion of each charging phase. This is because the charge acceptance varies continuously during a battery charging cycle, even within a particular phase. Thus, a constant level of input current, no matter how carefully selected, will not be optimum throughout an entire charging phase.